The conversion of methanol to olefins (MTO) is currently the subject of intense research because it has the potential for replacing the long-standing steam cracking technology that is today the industry-standard for producing world scale quantities of ethylene and propylene. The very large volumes involved suggest that substantial economic incentives exist for alternate technologies that can deliver high throughputs of light olefins in a cost efficient manner. Whereas steam cracking relies on non-selective thermal reactions of naphtha range hydrocarbons at very high temperatures, MTO exploits catalytic and micro-architectural properties of acidic molecular sieves under milder temperature conditions to produce high yields of ethylene and propylene from methanol.
Current understanding of the MTO reactions suggests a complex sequence in which three major steps can be identified: (1) an induction period leading to the formation of an active carbon pool (alkyl-aromatics), (2) alkylation-dealkylation reactions of these active intermediates leading to products, and (3) a gradual build-up of condensed ring aromatics. MTO is therefore an inherently transient chemical transformation in which the catalyst is in a continuous state of change. The ability of the catalyst to maintain high olefin yields for prolonged periods of time relies on a delicate balance between the relative rates at which the above processes take place. The formation of coke-like molecules is of singular importance because their accumulation interferes with the desired reaction sequence in a number of ways. In particular, coke renders the carbon pool inactive, lowers the rates of diffusion of reactants and products, increases the potential for undesired secondary reactions and limits catalyst life.
Over the last two decades, many catalytic materials have been identified as being useful for carrying out the MTO reactions. Crystalline microporous materials are the preferred catalysts today because they simultaneously address the acidity and morphological requirements for the reactions. Particularly preferred materials are eight-membered ring aluminosilicates, such as those having the chabazite framework type, and silicoaluminophosphates, such as SAPO-34 and SAPO-18. These molecular sieves have cages that are sufficiently large to accommodate aromatic intermediates while still allowing the diffusional transport of reactants and products into and out of the crystals through regularly interconnected window apertures. By complementing such morphological characteristics with appropriate levels of acid strength and acid density, working catalysts are produced. Extensive research in this area indicates that silicoaluminophosphates are currently more effective MTO catalysts than aluminosilicates. In particular, the control of the silica to alumina molar ratio is a key requirement for the use of aluminosilicates in MTO reactions, since materials with low silica to alumina molar ratios are too acidic and perform poorly. Nevertheless, aluminosilicate zeolites continue to be explored for use in MTO and appear to have yet undiscovered potential.
Chabazite is a naturally occurring zeolite with the approximate formula Ca6Al12Si24O72. Three synthetic forms of chabazite are described in “Zeolite Molecular Sieves”, by D. W. Breck, published in 1973 by John Wiley & Sons, the complete disclosure of which is incorporated herein by specific reference. The three synthetic forms reported by Breck are Zeolite “K-G”, described in J. Chem. Soc., p. 2822 (1956), Barrer et al; Zeolite D, described in British Patent No. 868,846 (1961); and Zeolite R, described in U.S. Pat. No. 3,030,181 (1962).
U.S. Pat. No. 4,544,538 describes the synthesis of another synthetic form of chabazite, SSZ-13, using N-alkyl-3-quinuclidinol, N,N,N-tri-alkyl-1-adamantammonium cations and/or N,N,N-trialkyl-exoaminonorbornane as a directing agent in a conventional OH− medium. According to the '538 patent, SSZ-13 typically has a silica to alumina molar ratio of 8 to 50 but higher molar ratios can be obtained by varying the relative ratios of the reactants in the synthesis mixture and/or by treating the zeolite with chelating agents or acids to remove aluminum from the zeolite lattice. However, attempts to synthesize SSZ-13 in OH− media at silica to alumina molar ratios in excess of 100 have been unsuccessful and have produced ITQ-1 or SSZ-23, depending on the alkali metal cation present. Moreover, increasing the silica to alumina molar ratio of SSZ-13 by dealumination has met with limited success because the small size of the pores makes aluminum extraction difficult and the severity of the treatment may detrimentally affect the crystalline integrity of the material.
Significant work has been conducted on the use of SSZ-13 as a catalyst for MTO reactions. However, investigations to date have shown that the performance of SSZ-13 is always inferior to that of its silicoaluminophosphate analog, SAPO-34. See, for example, Yuen, L.-T., Zones, S. I., Harris, T. V., Gallegos, E. J., and Auroux, A., “Product Selectivity in Methanol to Hydrocarbon Conversion for Isostructural Compositions of AFI and CHA Molecular Sieves”, Microporous Materials 2, 105-117 (1994) and Dahl, I. M., Mostad, H., Akporiaye, D., and Wendelbo, R., “Structural and Chemical Influences on the MTO Reaction: A Comparison of Chabazite and SAPO-34 as MTO Catalysts”, Microporous and Mesoporous Materials 29, 185-190 (1999).
Recently, a pure silica form of chabazite has been hydrothermally synthesized using N,N,N-trimethyladamantammonium in hydroxide form as the structure-directing agent at nearly neutral pH in the presence of fluoride. See Diaz-Cabanas, M-J, Barrett, P. A., and Camblor, M. A. “Synthesis and Structure of Pure SiO2 Chabazite: the SiO2 Polymorph with the Lowest Framework Density”, Chem. Commun. 1881 (1998).
By adding small amounts of aluminum to the synthesis mixture employed by Camblor and his co-workers, the present inventors have for the first time been able to synthesize a pure, highly crystalline aluminosilicate material having the chabazite structure with silica to alumina molar ratios significantly in excess of 100, such as 265. The amount of Al present in such a material is equivalent to only 0.045 Al atom/cage or 1 Al atom per 22 cages. Moreover, the results of this novel synthesis suggest that the Si/Al ratio can be controlled over wide ranges below and above 265.
MTO experiments have now been conducted with this high silica-chabazite at standard conditions of pressure, temperature, and space velocity and show very promising results. The performance of this catalyst is clearly better than any previous results on SSZ-13 and is only slightly inferior to those of the best low acidity SAPO-34 catalysts currently available. Lifetime curves for this catalyst as a function of temperature show fairly typical behavior of MTO catalysts in which an early induction period is followed by a high activity period and the eventual deactivation by coking. Despite the very small amount of aluminum in the catalyst, the activity is quite high, reaching 100% very early in the reaction.
It is to be appreciated that, although the chabazite of the present invention is normally synthesized as an aluminosilicate, the framework aluminum can be partially or completely replaced by other trivalent elements, such as boron, iron and/or gallium, and the framework silicon can be partially or completely replaced by other tetravalent elements such as germanium.